An ion selective electrode (ISE) is dipped into a sample solution together with a reference electrode to measure a potential difference between the two electrodes and thus determine ions, which are measurement targets, in the sample. Due to the simplicity, the ISE is widely used in the field of analysis. In the field of medicine, the ISE is used for clinical laboratory tests and is used to measure an electrolyte not only with a dedicated device for measuring electrolytes but also with a biochemical automatic analysis device, an urgent sample testing device, or a point-of-care testing device.
The ISE uses an ion-sensitive membrane to selectively measure ions, which are the measurement targets, from a plurality of ions. An ion-sensitive membrane that is typically used is a membrane obtained by mixing a plasticizer and a ligand into a polymer-based substance such as polyvinyl chloride, and by further mixing a salt thereinto depending on cases. The ion-sensitive membrane generates a potential based on the Nernst equation in accordance with the concentration of ions that are the measurement targets. Thus, if one surface of the ion-sensitive membrane is allowed to contact an aqueous solution with a constant concentration and the other surface is allowed to simultaneously contact a sample solution, it is possible to obtain electromotive force in accordance with the concentration of ions that are the measurement targets in the sample solution. The concentration of ions, which are the measurement targets, is calculated from the obtained electromotive force. Such an electrode with a structure in which a side of a sensitive membrane that is on the opposite side of a sample solution is filled with an aqueous solution or gel is called an internal-liquid-type ISE.
In contrast to the internal-liquid-type ISE, an electrode with a structure in which a surface of a sensitive membrane on the opposite side of a sample solution is allowed to be in direct contact with a conductive electrode is called a coated wire-ion selective electrode (CW-ISE) or a solid contact-ion selective electrode (SC-ISE). Herein, such electrodes are referred to as solid electrodes. Since solid electrodes do not require an internal solution, such electrodes can be easily reduced in size and have been applied to point-of-care testing devices. For an electrode (base electrode) that is allowed to be in contact with a sensitive membrane, platinum, silver chloride covered with a conductive polymer, gold modified with a monomolecular film, porous carbon, carbon black, or a mixture of a polymer and graphite has been used.